The present invention is especially well applicable for use in a cellular radio system that utilizes Code Division Multiple Access, CDMA. CDMA is a multiple access method based on spread spectrum technology, and it has recently been put to use in cellular radio systems, where FDMA and TDMA have been used before. An example for a known CDMA system is a broadband cellular radio standard EIA/TIA IS-95.
In a typical mobile phone environment, the signals between a base station and a mobile station usually propagate over multiple paths between a transmitter and a receiver. This multipath propagation is mainly caused by reflections of the signal from the surrounding surfaces. The multipath propagated signals arrive at the receiver at different times due to respective different propagation delays. In CDMA, multipath propagation can be used in the reception of signals in the same way as diversity. The autocorrelation characteristics of the spreading codes used in the transmissions make it possible to distinguish the different delay components from one another. For a CDMA receiver there is generally used a multibranch rake receiver in which each branch is synchronized with a signal component that has propagated over a different path. A digital reception unit comprises a plural number of rake branches, and each branch is an independent receiver element with the task of assembling and demodulating a received signal component. In a CDMA receiver, the signals of the different elements of a digital receiver unit are combined advantageously to obtain a high quality signal.
In CDMA systems, it is also possible to apply so called soft handoff, in which a mobile station may simultaneously communicate with a plurality of base stations utilizing macro diversity. The quality of the mobile station connection during the handoff is thus maintained high and the user does not notice a break in the connection. In the downlink direction (from base station to terminal equipment), two or more base stations send the same signal. Since the base stations use the same frequency, the terminal equipment may receive signalling from more than one transmitter at the same time. The signals from the different base stations are distinguished in the same way as the delay components from the different rake branches. In the uplink direction (from terminal equipment to base station), two or more base stations receive the same signal sent by the terminal equipment. The signals are combined in the first common point on the signal path. The soft handoff allows optimal power adjustment, which minimizes the interference level of the network and thereby maximizes the capacity of the network.
The performance of CDMA, which can be measured with spectrum efficiency, has been further improved by sectoring. A cell is divided into sectors of a desired size, and the sectors are served by directional antennas. The interference caused by the mobile stations to one another can then be significantly reduced in the base station receiver. The basis of this is that, on the average, interferences are evenly divided between different inlet directions, the number of which can be reduced by sectoring, as stated above. Sectoring can naturally be implemented in both directions of transmission. The capacity advantage brought about by sectoring is proportional to the number of sectors.
In a sectored cell, it is also possible to utilize a specific form of soft handoff called softer handoff, in which a mobile station performs handoff from one sector to another, communicating with both sectors at the same time. Soft handoff improves the quality of the connection.
Since the capacity of the CDMA is directly linked with the sensitivity of the receiver, the advantage achieved by any diversity method whatsoever will improve the overall capacity of the system. In addition to the above-described soft handoff, i.e. macro diversity, other diversity methods, such as place and time diversity, can naturally also be applied in connection with CDMA.
The present invention particularly concerns a structure of a base station receiver that optimizes the implementation of softer handoff. Prior art with respect to the structure of base station receivers is described in IEEE Personal Communications, Third quarter 1994, p. 28-34: `Reverse Link Performance of IS-95 Based Cellular Systems` by R. Padovani, which is incorporated herein by reference.
FIG. 1 illustrates a diversity receiver according to the prior art, the receiver serving, by way of an example, three sectors. In each sector, antenna diversity, i.e. two reception antennas, is used. Antennas 122, 124 serve the first sector, and antennas 126, 128 and 130, 132 serve the second and third sector, respectively. The receiver comprises a plural number of radio frequency units 100-110. Each antenna is connected to its own radio frequency unit, which converts the signal to an intermediate frequency and to digital form. The digital samples are transferred by means of an RX bus 120 to digital receiver units 114-118. The receiver further comprises a control processor 112, which controls the operation of the other components. To each receiver unit are guided the signals of one user, the signals optionally being received with various antennas or even from different sectors, where softer handoff is concerned. Bus 120 must thus be multiplexed so that the samples from different radio frequency units can be guided to correct reception units.
FIG. 2 illustrates the structure of a digital reception unit 114 of a diversity receiver according to the prior art. Each unit comprises demultiplexing means 200 that receive the desired samples and transfer them to demodulation means 202-206, each of which follows and demodulates one desired signal component. In the demodulation means, the spectrum is assembled by correlating a received signal with a spreading code, whereby the sample rate of the signal will be dropped by a spreading ratio. The thus assembled signal will then be demodulated. In IS-95, for example, uplink demodulation means a Walsh-Hadamard transformer in which the orthogonal signalling used is decoded, i.e. the signal that correlates the most with the input signal is selected. The unit further comprises searcher means 208, which follow and search for preferred signal components. The outputs of the demodulation means are supplied to a combiner 210, which combines the different signal components in an advantageous manner. A combined signal is supplied further to decoding means 212. The unit further comprises a control processor 214, which controls the operation of the other components.
In the solution of the prior art, a central problem is how to implement bus 120. In a base station according to EIA/TIA IS-95, for example, the sampling frequency, at which samples are taken in means 100-110, may be four- or eight-fold as compared with the chip rate (1.2288 Mchips/s) of the spreading code, i.e. 4.9152*10.sup.6 or 9.8304*10.sup.6 samples/s. Depending on the situation, the signal dynamics requires, for example, a 4 to 8 bit sample resolution, from which it follows that the total bit rate per one radio frequency unit is, at worst, up to 78.6432 Mbps (8-fold sample rate with an 8 bit resolution) and, at best, 19.6608 Mbps (4-fold sample rate with 4 bit resolution). These figures must be multiplied by the number of radio frequency units 110--110 connected to bus 120. Consequently, implementation of bus 120 is very expensive and technically demanding.